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Kinetics Problems Key
'Chemical Kinetics: Rates and Mechanisms Review Test ' 1. Expressing Reaction Rate Given: 3A + B à 2C + D Express the rate in terms of change in concentration of each product/reactant and change in time. A: -ΔA/3 C:{C}{C}{C}{C}{C}{C} D:{C}{C}{C}{C}{C}{C} 2. Reaction Orders Given: 2NO (g) + O2 (g) à 2NO2 (g) a. Determine the values of the Reaction orders (m and n). {C}{C}{C}{C} {C}{C}{C}{C}{C}{C} {C}{C}{C}{C} {C}{C}{C}{C}{C}{C} n=2 m=2 b. Determine the Rate Constant (k) Rate = kNO2O2 .05 = k(.01)2(.01) k = 50000 c. Write the Rate Law: Rate = 50000NO2O2 3. Label each of the following Integrated Rate Laws as zero order, first order, or second order: 1/At - 1/A0 = kt second order lnAt first order {C}{C}{C}{C} zero order 4. Consider the decomposition of Ammonia, which is a first order reaction with a half-life of 200 s at a certain temperature. How long will it take for the partial pressure to decrease from .100 atm to .00625 atm? (Hint: After one half-life, the partial pressure should be cut in half) A) 200 s Half-lives Remaining B) 400 s 0 .100 C) 800 s 1 .0500 D ) 1000 s 2 .0250 E ) 1200 s 3 .0125 4 .00625 200 s * 4 = 800 s 5. The following two steps are a proposed mechanism for the catalyzed oxidation of CO by O3: Step 1: NO2 (g) + CO (g) à NO (g) + CO2 (g) Step 2: NO (g) + O3 (g) à NO2 (g) + O2 (g) What are the overall products? CO2 + O2 6. Units of Rate Constant For a reaction with a rate law of Rate=kA2B What are the units of k? {C}{C}{C}{C} 7. Mechanisms Consider the following diagram: a. How many steps are there in this reaction? Two b. What are the initial reactants? Final products? Intermediates? A + B D + E C c. Which step is the rate determining step? Step 1 8. Catalysts Redraw the following diagram if a Catalyst had been used (there multiple ways to draw this): 9. Writing Rate Laws from a Proposed Mechanism Write the rate laws for the following elementary steps of a reaction: NO2 (g) + CO (g) à NO (g) + CO2 (g) Step 1: 2NO2 (g) à NO3 (g) + NO (g) Step 2: NO3 (g) + CO (g) à NO2 (g) + CO2 (g) Rate1 = k1NO22 Rate2 = k2NO3CO 10. Homogenous/Heterogenous Catalysts Label each of the following as Homogenous or Heterogeneous catalysts. A piece of metal reacting with gases in a closed container: Heterogeneous A substance is dissolved in methanol: Homogeneous A nickel compound is dissolved in HCN at 260 K: Homogeneous Silver metal is placed in a vial with an unknown liquid: Heterogeneous 11. For the simple decomposition reaction: AB (g) à A (g) + B (g) Rate = kAB2 k = .2 L/mol*s How long will it take AB to reach 1/3 of its initial concentration of 1.50 M? Second order reaction {C}{C} {C}{C} t = 6.7 = 7 s 12. Reaction Order Label each of the following as zero order, first order, or second order. t1/2 = {C}{C}{C}{C} second order t1/2 = {C}{C}{C}{C} first order t1/2 = {C}{C}{C}{C} zero order 13. 2HI (g) à H2 (g) + I2 (g) k1 = 9.51 * 10-9 L/mol*s at 500 K k2 = 1.10 * 10-5 L/mol*s at 600 K Using the data given, solve for the activation energy (Ea) of the decomposition of hydrogen iodide. {C}{C}{C}{C} {C}{C}{C}{C} Ea = 1.76 * 102 kJ/mol 14. Half-Life The rate constant for radioactive beryllium-11 is .049 s-1. After 28 s, how much mass of a 500 μg sample remains? A) 250 μg 28 s = 2 t1/2 B) 125 μg C) .0625 mg {C}{C}{C}{C} D) .375 mg E) .500 mg 15. Rate-Determining Step The slow rate of a chemical reaction might be attributed to which of the following? A) low activation energy B) high activation energy C) presence of a catalyst D) high temperature E) high reactant concentrations